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Introduction to Disinfection: From CT to UV
VA AWWA Plant Operations Committee Senior Operators Forum
Erik Rosenfeldt, Ph.D., P.E..
October 9, 2014Charlottesville, VA
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Agenda
• Background on Disinfection• Chlorine Disinfection• UV Disinfection• Summary and Conclusions
2
BACKGROUND
Source of some material: Water Treatment, MWH, 2nd Editionand Wastewater Engineering, Metcalf and Eddy – 4th Edition
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• Control of Waterborne Disease– Essential human need provided by
water engineers
• 20th Century– Control of Waterborne Diseases
(Typhoid, Cholera, etc) through Engineering• Through Treatment
Technologies• Role of the Engineer in Public
Health– Water Supply and Distribution– Wastewater treatment
Potential Health Issues
• Microbial Health Concerns– Cryptosporidiosis and Giardiasis
• Vomiting and diarrhea, potentially life threatening for immune compromised, elderly and young
• 1993 Milwauke outbreak 400,000 people got sick over 2 weeks– stomach cramps, fever, diarrhea and dehydration– 104 deaths attributed to the outbreak
• Disinfection By-Product Health Concerns– Cancer
• Bladder, colon and rectal– Reproductive
• Neural tube defects and miscarriages ?– Brominated compounds are thought to pose a greater health risk
than chlorinated compounds– Nitrogenated compounds may even be worse
5
Disinfection
• Goal– to destroy or inactivate pathogenic
microorganisms including bacteria, protozoan cysts, helminths and viruses.
– Pathogenic – disease causing
• Problem: all chemical disinfectants form unwanted byproducts
6
Infectious Agents Potentially Present in Untreated Domestic Wastewater
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Emerging Pathogens of Concern -Bacteria
8
Emerging Pathogens of Concern -Viruses
9
Emerging Pathogens of Concern -Protozoa and Algal Toxins
10
Types of Disinfectant Systems
• Chemical agents– chlorine (Cl2)– chlorine dioxide (ClO2)– ozone (O3)– chloramines
• Physical agents– UV irradiation– membranes
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Typical Water Treatment Plant Schematic
Raw Water Flocculation
Rapid Mix
Clarification Filtration
To System
Chemical Addition
Cl2NH 3
CorrosionFluoride
ClearwellNH3
Cl2ClO2KMnO4O3
Cl2ClO2KMnO4O3
Cl2ClO2KMnO4O3
UV
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Mechanisms of Disinfection
• Oxidation or rupture of cell wall• Diffusion into cell and interference with
cellular activity
Therefore, the ability to oxidize biological molecules and the ability to diffuse through the cell walls are the requirements of any effective disinfectant.
Chemical Disinfection (Oxidation) Mechanism
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Oxidizing Power of Various Oxidants
Oxidant Formula Oxidation Potential (V)Hydroxyl free radical oOH 2.80Ozone O3 2.07Hydrogen peroxide H2O2 1.76Permanganate ion MnO4
- 1.68Hypochlorous acid HOCl 1.49Chlorine Cl2 1.36Hypobromous acid HOBr 1.33Bromine Br2 1.07Chlorine dioxide ClO2 0.95Iodine I2 0.54Oxygen O2 0.40Hypochlorite ion OCl- <0.50
Note: italics denotes oxidants with excellent disinfection capabilitiesSource: AWWA, Chlorine Dioxide Handbook
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Disinfection Kinetics
• Principle theory:– Chick’s Law (1908) (Dr. Harriett Chick)
ln(N/No) = - kt (first order)
N - number of organisms present at time tNo - number of organisms present at time t=0k - rate constant, depends on:
> disinfectant type and concentration> microorganism> water quality (pH, turbidity, temperature)
t - time
ln(N/No)
t
k
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Disinfection Kinetics, cont.Later that same year, Watson refined the equation to include
changes in the disinfectant concentration:
k = k’Cn
ln(N/No) = -k’Cnt Chick-Watson Lawn- coefficient of dilutionk’- experimental constant
both n and k’ are determined experimentally
• when n>1, disinfecting action dependent on concentration• when n<1, disinfecting action depends on contact time
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Source: Berg, 1964 in JMM Book
Kinetics of Disinfection for Various Organisms
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Kinetics of Disinfection – Effect of Disinfectant Type
Source: Scarpino et. al.,, 1977 in JMM Book
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CT Concept
• C is the disinfectant residual concentration
• T is contact time. For regulatory purposes, we use the T10 time.
• T10 is determined from tracer study
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 10 20 30 40 50 60 70Time, minutes
C/Co
ln(N/No) = -k’Cnt
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CT ConceptChlorine pH <=6.0
Concentration Log Inactivations
(mg/L) 0.5 1.0 1.5 2.0 2.5 3.0<=0.4 23 46 69 91 114 137
0.6 24 47 71 94 118 141
0.8 24 48 73 97 121 145
1 25 49 74 99 123 148
1.2 25 51 76 101 147 152
1.4 26 52 78 103 129 155
1.6 26 52 79 105 131 157
1.8 27 54 81 108 135 162
2 28 55 83 110 138 165
2.2 28 56 85 113 141 169
2.4 29 57 86 115 143 172
2.6 29 58 88 117 146 175
2.8 30 59 89 119 148 178
3 30 60 91 121 151 181
• Go to CT tables in the SWTR Guidance Manual to find required CT
• Calculate residual required to meet CT requirements
• Function of chlorine dose, pH, and temperature
CT Values for Inactivation of Giardia Cysts by Free Chlorine at 0.5C or lower
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CHLORINE
Source of some material: Water Treatment, MWH, 2nd Editionand Wastewater Engineering, Metcalf and Eddy – 4th Edition
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Chlorine
• Most widely used disinfectant• Saved millions of lives and eliminated
waterborne diseases such as cholera and typhoid
• Relatively easy to use• Safety concerns for liquid chlorine cylinders or
tank cars (chlorine gas)• Relatively inexpensive• Major disadvantages:
– production of THMs, HAAs, other chlorinated disinfection byproducts
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Uses of chlorine (oxidant) in drinking water treatment plants
• Disinfection• Oxidizes color• Oxidizes iron and manganese• Taste and odor control• Controls aquatic growth in plants• Aid to filtration (particle removal)• Zebra mussel control
Raw Water Flocculation
Rapid Mix
Clarification Filtration
To System
Chemical Addition
Cl2NH 3
CorrosionFluoride
ClearwellNH3
Cl2ClO2KMnO4O3
Cl2ClO2KMnO4O3
Cl2ClO2KMnO4O3
Forms of chlorine
• Chlorine gas• Hypochlorite
1-ton chlorine cylinders
Sodium hypochlorite tanks
Chlorine gas
• Most commonly-used form of chlorine
• Effective disinfectant• Readily available• Normally most economical
method of disinfection• Decreases pH• Hazardous; must use care in
handling• Toxic to aquatic life (same for all
forms of chlorine)
Hypochlorite
• Usually in the form of NaOCl; also Ca(OCl)2
• Increases pH
• Higher cost than gaseous chlorine
Chlorine Speciation
0
10
20
30
40
50
60
70
80
90
100
5.0 6.0 7.0 8.0 9.0 10.0 11.0
pH
Perc
ent H
OCl
0
10
20
30
40
50
60
70
80
90
100
Perc
ent O
Cl
HOClOCl
HOCl ~ 100x stronger disinfectant than OCl-
Disinfection efficiency goes up with lower pH
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Reactions of chlorine with impurities in water
• Order of reactions– Inorganic compounds– Ammonia and certain organic nitrogen compounds– Organic compounds– Rates vary with compound and other conditions
Reactions of chlorine with impurities in water
• Reactions with inorganic compounds– Compounds which react quickly: hydrogen sulfide, ferrous iron,
MnII– Demand occurs before a chlorine residual occurs; must be
satisfied before any disinfection can take place
2Fe2+ + HOCl + H+ → 2Fe3+ + Cl1- + H2O
Reactions of chlorine with impurities in water
• Chloramines are formed when Cl2 reacts with ammonia (NH3)
NH3 + HOCl ↔ NH2Cl + H2O monochloramine
NH2Cl + HOCl ↔ NHCl2 + H2O dichloramine
NHCl2 + HOCl ↔ NCl3 + H2O trichloramine(nitrogen trichloride)
Breakpoint Chlorination CurveC
hlor
ine
conc
entr
atio
n
InitialChlorineDemand
BREAKPOINT
Free ChlorineResiudal on a1 to 1 Basis
Free ChlorineResidual on a1 to 1 Basis
ChloraminesChloroorganics
Chlorine Dose, mg/l
CombinedResidualChlorine
InorganicCompoundsReducing Agents
Oxidation of CombinedResidual Material(Chloramines)
Oxidation of chloramines to N2 or NO3
-; decreasing combined residual
Chloramines for Disinfection
• Not effective for Cryptosporidiuminactivation
• Used to provide a residual in some distribution systems
• “Halts” the formation of THMs or HAAs• May control biofilms/regrowth better than
free Cl2• Relatively inexpensive
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Reactions of chlorine with impurities in water
• Reactions with organic compounds– Occur after reactions with inorganic compounds– Produce chlororganic compounds or other combined forms of
chlorine– Have slight disinfecting action
• Free chlorine residual– Produced after all other above reactions– Highest disinfecting capability– Rarely exists in wastewater with nitrogenous compounds.
Amount of chlorine required would be 25 to 150 mg/L.
Chlorine dose
Chlorine Dose = Chlorine Demand + Chlorine Residual
Where:Chlorine Residual = Combined Chlorine + Free
Chlorine– Contact time needed for disinfection– Contact time must be specified since longer contact times
increase chlorine uptake (decrease chlorine residual)
UV DISINFECTION
Source of some material: Water Treatment, MWH, 2nd Editionand Wastewater Engineering, Metcalf and Eddy – 4th Edition
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Electromagnetic Spectrum 36
Mercury Vapor UV Lamp Spectra37
Ultraviolet Disinfection
• Uses lamps submerged in water that emit light at 254 nanometer wavelength
• UV light keeps pathogens from reproducing by affecting their DNA and RNA
• Killing effectiveness depends on the intensity of light and the time in contact with the microorganisms
• More effective than chemical oxidants for resistant organisms– Cryptosporidium– Giardia
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Comparing UV and Chlorine Disinfection
Parameter UV Disinfection Chlorine Disinfection
ActionMechanismResulting Cell StructureReactivationDesign DoseCalculated DoseExposure TimeResidualBackground Demand
Effectiveness
PhysicalDNA DamageIntactPhoto/darkFixed/semi-variableI x TSecondsNoneAbsorbanceAttenuationWavelength
ChemicalOxidationDamagedResuscitationAdjustableC x TMinutesVaries with chlorine demandOrganics/inorganicsSunlightpH
• Based on Table 18-1 in AWWA WQ&T
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UV Disinfection: Mechanism of Action
AC
GTAACTT A
G
G C
T
UV
DNA
• Physical Process– Light Energy Absorbed by
DNA– Pyrimidine Dimer
Formation (C’s and T’s)
• Inhibits Replication– Organism that Cannot
Replicate, Cannot Infect
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UV Disinfection is Dependent on Microorganism
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• Bacteria can repair the damage caused by UV light– Photorepair
• enzymes energized by exposure to light break the pyrimidine dimers.
– Dark repair is any repair process that does not require the presence of light.
• Example: Excision Repair
ACGTAACTT A
GG C
T
VIS
DNA
Microbial Repair
ACGTAACTT A
GG C
TDNA
Ex. Dark RepairEx: Light Repair
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Effectiveness of UV -Wavelength
DNA Absorbance
43
WATER QUALITY IMPACTS ON UV EFFECTIVENESS
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UV Absorbance (Transmittance)
• Stuff in water other than microorganism (target) that can absorb UV light
• UV decays exponentially through a medium– I = Io x 10-αl
• α = background absorbance• l = distance from source
• Absorbance and Transmittance are related– %UVT = 100x10-A
Particles
• Problem – Particles “shielding” microorganism from UV
• Solution: Multiple light sources?
46
Particles
• Reality: particles are not spheres, and microorganism are very small
• Solution: Removeparticles withfiltration prior to UVdisinfection
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Lamp Fouling
• Materials in water can deposit on the quartz sleeve– Hardness– Organics– Iron
• Affects Intensity of light entering reactor
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Hydraulics are extremely important
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Summary
• Drinking water disinfection is a key component of public health protection.
• There are various disinfection chemicals and applications available to utilities
• Selection of disinfection techniques is system specific
• Disinfection can lead to the formation of DBPs in the system
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Acknowledgements
• Dr. James K. Edzwald• Bill Becker and Julie Herzner, Hazen and
Sawyer
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